It's all about consistency if you ask me. If you're frosting is too thin (or too warm), your grass will fall over. If it's too stiff, you'll have short stubby grass that looks more like a buzz cut. Practice first to check the consistency and you'll have fun. It's a very forgiving technique (I'm the worst at piping and even I can do it!)

Yep. Consistency. Also, as you pipe, your hand will warm the icing in the bag, making it very soft. I suggest having two or more piping bags made up so you can put the warm one in the frig for a few minutes and continue working with the other bag.

I like to marble the buttercream with different shades of green. It gives a more realistic look.

Base ice your cupcakes with a thin coat of green icing so any areas that don't get covered with grass won't show up. If you try it without base icing, you'll see what I'm talking about.

I also suggest buying a backup grass tip. Sometimes a few of the holes will get clogged. Instead of having to waste decorating time washing the tip, just switch to a clean one. (I have 6 grass tips!)